10 is a difficult age.. well heck I am 37 and its still not easy..
everyone so far as given you some great advice.. Its great that you are being so accepting of this... Its not something that comes easy to talk about with your parents at any age especially as a child. I will agree with some of the other in just getting him some of his own girls clothes. Letting him pick it out is a good idea but going to the store might be a bit much for him at this age. Its taken me years to realize that the cashiers really dont care what you buy. Perhaps on one of your regular shopping trips to buy him other clothes you can pick up a few girl things. I know its been said before but I think it bears repeating.. dont treat him any differently than you would if he werent wearing girls clothing..
Its good that your trying to find out so much more about this. I know many cders probably wish they would have had understanding parents when they were children.